1. Field of the Invention
The present invention relates to a display device having pixels formed of self-luminous elements such as organic electro-luminescent (EL) elements.
2. Description of the Related Art
In recent years, attention has been attracted to flat-panel display devices such as an organic EL display device, and active research and development have been made for use in a personal computer, portable information terminal, and the like. An organic EL display device has an advantageous feature that does not employ a backlight undesired for reduction in thickness and weight, is suited for reproduction of moving images owing to its high-speed response, and is usable in a cold place because a decrease in the luminance is not caused by a low temperature.
Such an organic EL display device generally comprises a matrix array of pixels each having an organic EL element that emits light at luminance corresponding to a current supplied thereto and a driving circuit for driving these pixels. The driving circuit includes a digital-to-analog (D/A) conversion circuit for converting, for example, digital pixel signals into analog pixel signals and a gradation reference circuit for generating a plurality of gradation reference voltages or currents that are referred to by the D/A conversion circuit. In a case where the gradation reference circuit divides, for example, a reference power supply voltage to generate these gradation reference voltages, the D/A conversion circuit selects one of these gradation reference voltages based on the digital pixel data signal and outputs the selected voltage as an analog pixel signal. Each pixel is driven according to the analog pixel signal obtained in this manner.
In this organic EL display device, brightness of a display screen depends on currents flowing in the organic EL elements. In the case where the organic EL elements are set at the maximum luminance to obtain a gradation of white on the entire display screen, a considerable amount of power is consumed according to the total sum of currents flowing in the organic EL elements. Further, such power consumption requires a power supply circuit that has a current supply capacity suitable for the currents flowing in the organic EL elements, thus leading to an increase in manufacturing costs and outer dimensions of the power supply circuit.
These problems should be remedied for organic EL display devices to be incorporated in equipment where the manufacturing cost, power consumption, volume, or the like is restricted. Conventionally, the luminance for the white gradation has been restricted to reduce currents flowing in the organic EL elements. However, this gives the impression that a white display portion is darkish when the area of the white display portion becomes small in the entire display screen.